Hydraulic jacks and the like



Aug.v29, 1967 ca. PERSICKE HYDRAULIC JACKS AND THE LIKE Filed April '23, 1965 5 Sheets-Sheet l T m I m m 9 m m 0 Inventor GUNTER PERsicKE B Z 4 YM a Attorney 5 29, 1 6 G. PERSICKE HYDRAULIC JACKS AND THE LIKE Filed April 23, 1965 5 Sheets-Sheet Invenlor Aug. 2 1 G. PERSICKE HYDRAULIC JACKS AND THE LIKE 5 Sheets-Sheet Filed April 23, 1965 Inventor GUNTE'R PERsicKE ZWLI. *M;

A ttorneys Aug. 29, 1967 s. PERSICKE HYDRAULIC JACKS AND THE LIKE 5 Sheets-Sheet 4 Filed April 23, 1965 lnvenlor 'PERSI'CKE G UNTER y; @M Attorney;

Aug. 29, 1967 3,338,552

HYDRAULIC JACKS AND THE LIKE Filed April 23, 1965 5 Sheets-Sheet s Inventor GUNTER "PVERS ickE United States Patent G 3,338,552 HYDRAULIC JACKS AND THE LIKE Gunter Persicke, Dartford, Kent, England, assignor t Christiani & Nielsen Limited, London, England, a corporation of Great Britain Filed Apr. 23,1965, Ser. No. 450,311 12 Claims. (Cl. 25429) ABSTRACT OF THE DISCLOSURE Hydraulic jacks of relatively small compass and having a short stroke but of great power for prestressing concrete reinforcing rods. The jacks have annular pistons and fluid pressure-type expansible sealing rings.

Summary In the manufacture of prestressed concrete elements such as beams or pipes, the reinforcing rod-like elements of steel are placed in a mould and fixed at or towards one end of the mould and the other end extends outside the mould. A jack is held against the outer end of the mould or a member connected thereto and fixed to the rod-like element; the jack is then operated to exert a longitudinal pull on the rod to put it in tension and then a fixing member or anchor is applied to the rod and against the outer end of the mould or the member associated with it. The concrete is poured into the mould while the reinforcing element is under tension. The jack is usually removed from the reinforcing element during the pouring and resecured in position, when the concrete has set, to remove the initial stress on the reinforcing element and in some cases to permit the element to be removed from the concrete element.

The jacking load applied to the reinforcement element is in the order of 60 to 70 tons and to apply such a pressure the jacks of known construction have to be of large proportions which is frequently undesirable in the space available and also they are heavy and often cumbersome to use.

The main object of the present invention is to provide an hydraulic jack, more especially but not exclusively for prestressing reinforcement elements in the manufacture of prestressed concrete constructional elements.

According to the present invention an hydraulic jack comprises an annular cylinder having a pressure fluid chamber in the head end portion thereof with a pressure fluid inlet thereto, an annular piston slidably disposed in the cylinder and having at its outer end accessible from the outside of the cylinder a portion by which the jacking action is effected, inner and outer hollow piston sealing rings between the piston head and said portion thereof and surrounding the inner and outer circular faces of the piston respectively in sliding engagement with the inner and outer circular walls of the interior of the cylinder, and passages in the piston head providing communication between the pressure fluid chamber and the hollow interiors of the sealing rings to allow the pressure fluid to fill the interiors of the rings and ensure a fluid-tight seal of the rings with the piston and the cylinder walls.

Preferably the piston head is formed by a cover plate secured to the body of the piston and the rings are secured in position round the piston by said cover plate, the plate having the said passages therein. The rings may be U-shaped and the outer and/or inner edge of the cover plate may be chamfered to form a firm seat on the outer and/or inner edges of the U-shaped rings. A bearing member which may be a hollow element or annulus may be provided for insertion between the head and an element, such as the end of a concrete constructional element, to which a thrust is to be imparted by the jack.

In one construction the cylinder is cylindrical and has a central ,axial cylindrical boss therein forming the annular working chamber, the inner piston ring sealing against the surface of the boss. The boss may have an axial bore therethrough or be formed hollow so that a rod-like member to be jacked may be passed therethrough. The outer end of the boss may be provided with a cover plate fixed thereto, and a duct leads through the boss from the cylinder working chamber so as to be uncovered by the piston when at the outer end of its working stroke, the duct opening to exhaust, or into the boss interior when hollow, so that the pressure in the cylinder is reduced at the end of the working stroke of the piston.

The hollow boss may have a tapered interior and a number of wedge segments are disposed therein to engage round a rod-like member passed through the boss to grip the member as the jack exerts an axial force therealong. The boss in this construction has its inner peripheral portion in axial alignment with the segments, and the bore in the boss communicates with chambers in the boss cover plate, and pistons are slidably disposed in the boss chambers to be projected therefrom to engage and exert an axial pressure on the segments when the piston reaches the outer end of its working stroke thereby increasing the grip of the segments on the rod-like memher.

In another embodiment the piston is provided within the cylinder with one or more axially directed minor pistons each slidable in a cylindrical bore in the cylinder head, and the head has a plurality of extra jacking cylinders in each of which slides a piston having a projection extending out of the head to engage an element against which a thrust is to be imparted, there being a fluid flow path from the jack cylinder through at least one of said cylindrical bores to the extra jacking cylinders such that when the jack reaches a predetermined jacking displacement at least one minor piston moving in its cylindrical bore will open said flow path and permit the pressure of the jack cylinder to be applied to the extra jacking cylinders and force the said projections to engage said element and apply-a jacking thrust thereagainst.

In order that the invention may be more clearly understood some embodiments in accordance therewith, as used for prestressing the reinforcement element of a concrete constructional member, will now be described by way of example with reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic longitudinal cross section showing the employment of the jack in prestressing the concrete constructional member;

FIGURE 2 is a cross section on an enlarged scale through the jack;

FIGURE 3 is a plan view of the piston along the line III-III of FIGURE 1; and

FIGURE 4 is a detailed view of the boss in the cylinder;

FIGURE 4a is a vertical cross section through another embodiment;

FIGURE 5 is a cross section on the line VV of FIG- URE 4a looking in the direction of the arrows;

FIGURE 6 is a vertical cross section through a further embodiment;

FIGURE 7 is a cross section on the line VIIVII of FIGURE 6 looking in the direction of the arrows;

FIGURE 8 is a cross section through yet another construction;

FIGURE 9 is a cross section on the line IXIX of FIGURE 8 looking in the direction of the arrows; and

FIGURE 10 which is a vertical cross section through another embodiment.

In the drawings the same references have been used to designate the same parts.

Referring to FIGURE 1, this shows a mould 1 in which a precast prestressed concrete unit is constructed, the mould having end walls 2, 3. A reinforcement element in the form of a steel rod 4 is fixed in the mould by an anchor 5. The anchor 5 is shown as an enlargement on the rod but it may be anchored within the mould towards the end wall 2, or in any other suitable maner. The free end 8 of the rod is screw threaded to receive an anchor nut 9 in the manner to be described herein. The jack 10 is mounted on the end 8 of the rod 4 and is spaced from the mould 1 by members 11, which may form a cage, so that a final anchor 12 on the rod can be engaged against the mould wall 3 after the rod has been prestressed by the jack the piston 13 of which moves axially with respect to the cylinder 14 to force the nut 9 on the rod to the left in FIGURE 1.

Referring to FIGURE 2, this shows the cylinder 14 of the jack in which slides the piston 13. The cylinder has a head 15 with an outside wall 16 and a central boss 17 which enclose an annular working chamber 18, with an hydraulic fluid inlet 19 in the head end portion thereof. The outer end 20 of the piston is shaped, in the drawings with a flat face, for pressing against a part to be loaded or lifted namely the rod 4 in FIGURE 1.

The piston 13 has a portion 21 in sliding engagement with the inside face of the outer cylinder wall and from the portion 21 an annular flange 22 extends axially, around which inner and outer sealing rings 23, 24 are fitted with their open edges 25 facing, preferably, towards the cylinder head as shown. The rings are secured to the piston by a cover plate.26 forming part of the piston and fixed as by screws 27 to the flange 22 and having ducts or holes 28 through which the pressure fluid in the chamber 13 flows into the hollows of the rings 23, 24 and forces their flange parts into a fluid-tight seal with the contiguous surfaces of the flange 22 and the opposed faces of the annular chamber 18. The outer (and/or inner) edge of the plate 26 is preferably chamfered as shown in FIGURE 2 to form a firm seat on the edges of the rings 23, 24.

Thus the pressure of the working fluid which may be for example 8,500 to 9,000 pounds per square inch is applied to the rings 23, 24 which form a highly satisfactory seal thereby permitting the high pressures employed with small piston area/thrust ratio and consequently a low weight/thrust ratio.

The length of the cylinder and the piston stroke may be constructed to suit the purpose for the pack, but in one construction a cylinder length of 2 /2 inches with outside radius of 6 inches and a stroke of inch are used.

It is frequently desirable to employ a safety device to prevent overload of the piston at its outer end of the stroke and for this purpose a duct 280 is provided in the wall of the boss 17, with its inlet 29 from the chamber 18 disposed so as to be uncovered by the piston at the outer end of its stroke, the duct leading at 25 into the hollow of the boss so that there is a relief of pressure preventing any elongation of the piston stroke. Several su-ch ducts may be provided of varying cross section so as to permit a progressive relief of the pressure in the chamber 18.

For ordinary lifting purposes the jack can be placed with its head on the ground and the load to be lifted placed on the outer face of the piston, but for use in prestressing concrete construction members reinforcing elements as in FIGURE 1 the jack is constructed with the boss 17 shaped as will now be described.

The boss 17 is hollow and there is a central aperture 30 in the cylinder head as well as a bore 31 in the piston. The hollow boss has a tapered interior face 32 and a number of tapered segments 33, in the drawings four in number as an'example, are freely located within the boss so as to engage round the rod 4 which is passed through the centre of the jack. The segments may have as shown threaded inner faces 34 to engage the thread on the rod 4. Thus when the jack is charged with high pressure fluid causing relative movement between the cylinder and the rod, the segments already clamping the rod are more tightly gripped against the rod by the camming action of the taper.

This gripping action on the rod 4 may be further augmented by the flow of fluid through the duct 280 in the boss. For this purpose a cover plate 35 is screwed into the boss 17, the plate 35 having a number of chambers or extra jacking cylinders 36 disposed therearound, at least one for each segment 33, in which a small piston 37 having a projection slides out of the plate 35 towards the segments. When the inlet 29 to the duct 280 is uncovered by the piston fluid flows through the duct 280 into a groove 38 round the cover plate 35 into the cylinders 36 and forces the pistons 37 rapidly into engagement with the segments 33 urging them axially in the boss so that by the action of the boss taper their grip on the rod 4 is increased.

The rod 4 is thus elongated by increasing the pressure in the working chamber in the jack. At the outer limit of the working stroke of the piston, the opening 29 is uncovered and the projections on the pistons 37 are forced against the segments 33 which are forced inwardly to grip the rod 4 firmly and prevent it slipping through the jack. When the jack is depressurised and the nut is tightened against the piston 13 the jack may again be pressurised to force the piston 13 to the outer end of its stroke and this cycle can be repeated until the required load has been applied to the rod 4, at which stage the anchor 12, e.g. a nut screwed on the rod, is tightened against the mould wall to secure it in tension while the concrete is poured into the mould. Alternatively the anchor 12 may be fixed, after each extension of the jack when the jack is depressurised and the operation repeated with the segments repositioned on the rod 4. Referring to FIGURES 4a and 5 these show another embodiment in which can be seen the cylinder 14, piston 13, piston rings 23, 24, cylinder boss 17, apertured .plate 26, fluid inlet 19 and other parts as already described with reference to FIGURES l to 4. The cylinder head 15 is flat and can rest on a firm support 39 while the load to which a thrust is to be imparted is supported on the head 20 of the .piston, or the jack may be inverted so that the piston head is on the firm support 39 and the load is supported on the cylinder head 15 in which case the cylinder will move axially with respect to the piston during jacking.

Referring to FIGURES 6 and 7 the reference numerals corresponding to the similarly numbered parts in FIG- URES l to 3 can be identified. The cylinder 14 has however its lower end of stepped shape at 40 and fits into a corresponding shaped head 41 of .a bearing member 42 which is shown resting on an endplate 43 set into the end of a concrete constructional element 1. The bearing memher is shown as annular and the piston 13, the cylinder boss 17 and the head 41 of the bearing member are provided with axial passages in register through which the reinforcing rod 4 passes. The nut 9 on the threaded end of the rod 4 is screwed down onto its washer 44 which engages the piston head 20 and the further nut 12, also screwed on the rod 4, is screwed down onto a washer 46 which engages the endplate 43. When the jack is operated with a fluid pressure, of say 8000 to 9000 pounds per square inch, the piston slides up in the cylinder 14 and extends the rod 4, the nut 12 then being screwed along the rod 4 until the washer 46 is again forced against the end plate 43; the jack can then "be returned to zero pressure so that the piston 13 slides fully into the cylinder 14 and the jacking operation can then be repeated thereby tensioning the rod 4 to the desired degree. The annular wall 11 of the bearing plate has apertures 47 as shown to give ready access of a tool such as a spanner to the nut 12. With this construction the jack can be removed after the jacking operation and the bearing member 42 can be removed and re-used elsewhere, only the plate 43 remaining permanently on the pile.

Referring to FIGURES 8 and 9 this again shows parts comparable to similarly numbered parts in FIGURES 2 and 3. The cylinder head 15 engages on an end plate 43 mounted on the element end 1. Conical segments 33 are disposed in a tapered bore 48 in the end plate 43 so that the tension in the rod 4 will force the segments in the bore 48 thus firmly gripping the rod 4 as described with reference to FIGURE 1.

The cylinder head 15 has one or a number (as in FIG- URE 8) of cylindrical bores 49 into each of which a minor piston 50 on the piston 13 slides, the pistons having tapped ends 51 used in place of the screws 27 of FIGURES 1 to 7 for holding the plate 26 in position on the piston flange 22. A fluid flow path leads from the cylinder 14, through a duct 52 into a bore 49 and thence through a duct 53, into extra jacking cylinders 54 in a ring 55 screwed into a recess in the piston head 15. Each cylinder 54 has a piston 56 sliding thereon with projections 57 extending through apertures 58 in the ring 55.

In operation, when the jack is not pressurised by fluid, the piston flange 21 is fully within the cylinder 13 and the pistons 50 are in the bores 49 so that the fluid flow path through the bores 49 is closed and the pistons 56 are withdrawn into their cylinders. As the pressure fluid flows into the cylinder 14 the piston 13 rises jacking up the load (i.e. tensioning the rod 4) and in due course the pistons 50 uncover the ducts 52, 53 whereupon fluid flows to the cylinders 54 to force the pistons 56 axially thereon and the projections 57 are forced through the apertures 58 into engagement with the segments 33 pressing the segments into tighter engagement with the rod 4.

The jack may be used for tensioning a rod 4 which is passed through a duct cast in the already cast concrete member, in which case the rod is anchored to an anchor in the member or at its outer end and the jack or members 11 bear against the end of the concrete member itself.

Although the rings 23, 24 make a tight fluid seal with the cylinders, boss and piston, their resilience may permit some rocking or canting of the piston in the cylinder and the rings will still maintain the fluid-tight seal, which is useful where the load is not evenly distributed around the longitudinal axis of the jack. The anchor 12 may be of any desired construction.

Moreover segments 33 of different sizes may be employed so that by selecting a set of a particular size the jack can be quickly adapted to engage rods 4 of varying diameters.

Referring to FIGURE 10, the jack is of similar construction to the jacks described with reference to FIG- URES l to 9 and comprises an annular cylinder 14 in which is slidably disposed an annular piston 13, the cylinder having a hydraulic fluid inlet 19 in the head end portion thereof. The piston 14 has, at its outer end accessible from the outside of the cylinder, a portion 20 by which the jacking action is effected. Inner and outer 1101- low sealing rings 25 and 26 respectively surrounding an annular flange 22 of the piston are in slidable engagement with the inner and outer walls of the interior of the cylinder and passages 28 are provided in the piston head leading from the pressure fluid chamber 18 of the cylinder to the hollow interiors of the rings 25 and 26, thereby allowing pressure fluid to fill the hollow rings and ensuring a fluid-tight seal of the rings with the piston and cylinder walls.

The prestressed concrete unit 1, for example, having a reinforcement element which is to be prestressed by the jack is shown in the form of a steel rod 4 fixed in the element by an anchor through which it passes, but which may be any other suitable anchoring means. The free end 8 of the rod is threaded to receive an anchor nut 9 and the jack is mounted on the end 8 of the rod 4, being spaced from the rod and the element by the member 11. The rod is prestressed by the piston 13 moving axially with respect to the cylinder 14 to force the nut 9 in a direction away from the anchor 5 whereupon a further anchoring nut 12 is screwed along the rod 4 towards the anchor until it engages there against. The outer portion of the piston is shaped, in the embodiment shown, with a flat face against which the nut 9 transmits the load exerted by the jack to the part to be stressed or lifted i.e. the rod 4, through the anchoring nut 12.

Each of the rings and 26 is secured to the piston by a cover plate 32 fixed as by screws 27 to the flange 22 and the ducts or holes 28 are formed in the cover plate. Presssure fluid in the chamber 18 flows through the holes 28 into the hollows of the rings 25 and 26 and forces their flange parts 41 into fluid tight engagement with the flange 22 and the inner face of the outer cylinder wall.

A duct 44 is provided in the flange 22 between the chamber 18 and a cavity 42 opening into the cylinder wall in which is disposed a valve body 43 having a bore 46 normally closed by a ball 47 on a seat 48 against which it is pressed by a spring 49. A rod or stem 50 is slidably disposed. in the bore 46 and urged by the spring to extend out of the bore and engage the cylinder wall.

The cylinderwall has a recess 51 the bottom of which at its edges merges smoothly with the cylindrical cylinder wall, and the recess is disposed so that in the inner position of the piston the stem 50 enters and engages the bottom of the deepest part of the recess.

Fluid entering the chamber 18 passes through the duct 44 into the cavity 42 augmenting the force of the spring to force the ball 47 onto the seat 48 and force the stem 50 outwardly of the piston 13, moves outwardly in the cylinder and the outer end of the stem 50 rides out of the recess 51 and forces the ball of seat 28 so that pressure fluid will escape through the duct 44 into the lower pressure zone formed by the recess in communication with the atmosphere thus preventing the pressure in the annular cylinder from increasing further and thereby maintaining the annular piston in a given jacking position.

The assembly can be constructed to allow the annular piston to move to any predetermined position by correctly designing the recess 51 which may run round the cylinder wall or extend over part of the wall, means being provided to restrain rotary movement of the piston in the cylinder.

The duct 44 and cavity 42 with the valve is shown in the piston, but it will be understood that it may be disposed in the outer cylinder wall, with the recess in the piston wall.

It will be seen that in FIGURE 10 the pressure beneath the jack piston 22 is relieved, preventing further outward movement of the piston in the jack cylinder before the jack piston can move out of the jack cylinder or approach the outer end of the jack cylinder thereby avoiding any damage to the loaded jack or the member under load from the jack. This safety arrangement in FIGURE 10 may be used in any of the jacks shown in FIGURES 1 to 9.

I claim:

1. An hydraulic jack comprising an annular cylinder, a pressure fluid chamber in the head end portion of said cylinder, a pressure fluid inlet to said chamber, an annular piston slidably disposed in said cylinder, a portion of said piston being accessible outside said cylinder by which the jacking action is effected, inner and outer hollow sealing rings of U-shaped cross-section between said portion and the head of said piston respectively surrounding the inner and outer walls of said piston and in sliding engagement with the inner and outer walls of said annular cylinder interior, and a circular cover plate fixed to the body of said piston and securing said rings round said piston, said plate having passages therein providing communication between said chamber and said hollow interior of said rings to force said rings under pressure into fluid-tight engagement with said cylinder and piston walls, and at least one of the outer and inner edges of said plate being chamfered to provide a firm seat on at least one of the outer and inner edges of said rings.

2. Au hydraulic jack comprising a cylinder, a central axial boss in said cylinder forming an annular pressure fluid chamber, an annular jacking piston slidably disposed in said chamber, a pressure fluid inlet to said chamber above the cylinder head therein, a portion on said piston remote from its head and accessible outside said cylinder by which jacking is effected, inner and outer hollow sealing rings between said portion and said head surrounding respectively the inner and outer walls of said piston and in sliding engagement with the Walls of said boss and said cylinder interior, passages in said piston head providing communication between said chamber and said hollow interior of said rings to force said rings under pressure into fluid-tight engagement with said cylinder, boss and piston walls, and a cover plate fixed to the outer end of said =boss, said boss having a duct leading from said chamber to be uncovered by said piston when at the outer end of its working stroke and opening to exhaust, whereby the pressure in said chamber is reduced when said piston reaches the end of its working stroke.

3. A jack according to claim 2 wherein said boss has a tapered axial bore extending therethrough, a number of wedge shaped segments are disposed side by side in said bore with an axial gap through said segments, a cover plate is fixed to the outer end of said boss, a duct leads through said boss from exhaust to said chamber so as to be uncovered when said jacking piston is at the outer end of its working stroke, said cover plate having its inner peripheral portion in axial alignment with said segments, chambers in said cover plate in communication with said duct, pistons slidably disposed in said chambers and projectable therefrom to engage and exert axial pressure on said segments when the jacking piston reaches the outer end of its working stroke to increase the grip of said segments on a rod-like member to be jacked passing through said boss bore and said gap between said segments.

4. An hydraulic jack comprising an annular cylinder, a pressure fluid chamber in the head end portion of said cylinder, a pressure fluid inlet to said chamber, an annular piston slidably disposed in said cylinder, a portion of said piston being accessible outside said cylinder by which the jacking action is effected, inner and outer hollow sealing rings between said portion and the head of said piston respectively surrounding the inner and outer walls of said piston and in sliding engagement with the inner and outer walls of said annular cylinder interior, and passages in said piston head providing communication between said chamber and said hollow interior of said rings to force said rings under pressure into fluidtight engagement with said cylinder and piston walls, at least one cylindrical bore in the head of said cylinder, said annular piston having at least one minor piston in said cylinder and slidable within said at least one cylindrical bore, at least one extra jacking cylinder in said cylinder head, at least one extra jacking piston slidable in each said extra jacking cylinder, a projection on each extra jacking iston extending out of said head to engage a member on the element to be jacked, a fluid flow path through said at least one bore to said at least one extra jacking cylinder such that when said annular piston reaches a predetermined point in its outward movement each said minor piston moves in its said bore to open said flow path to cause said projection to apply a jacking thrust to said element.

5. A jack according to claim 4 wherein said flow path comprises ducts in said cylinder at least one of said ducts leading from said jack cylinder to at least one of said bores and at least one other of said ducts leading from each said bore to one of said extra jacking cylinders.

6. A jack according to claim 4 wherein the extra jacking cylinders are provided in an element fast with the outer face of said cylinder head.

7. An hydraulic jack, an annular cylinder element, a pressure fluid chamber in the head end portion of said element, a pressure fluid inlet to said chamber, an annular piston element slidably disposed in said cylinder element, a portion of said piston element accessible outside said cylinder element by which the jacking action is effected, inner and outer hollow sealing rings between said portion and the head of said piston element respectively surrounding the inner and outer walls of said piston element and in sliding engagement with the inner and outer walls of said cylinder element interior, passages in said piston element head providing communication between said chamber and the hollow portions of said rings to force said rings under pressure into fluid-tight engagement with the said walls of said elements, a duct between said chamber and a recess in the wall of one of said elements, a valve in said duct, a valve member associated with said valve urged into engagement with the wall of said recessed element in such manner that as the piston element approaches the end of its working stroke under pressure said valve member enters said recess and operates said valve to permit pressure fluid to escape to exhaust through said duct and recess and thereby prevent further continued movement of said piston element.

8. A jack according to claim 7 wherein said duct is in said piston element, a cavity is provided in the wall of said piston element in communcation with said duct and the exterior of said piston element wall, said valve controls flow of fluid in said duct to said cavity and said valve member comprises a valve stem extending from said cavity with its end in engagement with the adjacent wall of said cylinder element and said recess therein.

9. A jack according to claim 7 wherein said valve comprises a valve body in one of said elements, a valve seat normally closed by a spring loaded ball, a valve stem extending from said ball through said body and protruding therefrom, slidably to engage the wall and recess in the other of said elements.

10. A hydraulic jack comprising, in combination, a cylinder having a base for reacting against a surface, an outer wall outstanding from said base and having an inner cylindrical surface, a boss outstanding from said base and having an outer cylindrical wall concentric with said inner cylindrical wall to define therewith an annular pressure fluid chamber, a piston having a head for reacting against a surface and an outstanding annular skirt slidably received in said pressure fluid chamber, means for introducing pressure fluid into said pressure fluid chamber, sealing means carried by said skirt of the piston and engaging said inner surface of the outer wall of the cylinder and said outer cylindrical wall of the boss for sealing against the loss of pressure fluid past the piston, said sealing means comprising a pair of rings, each of V-shaped cross section defining inner and outer lips, one of said rings having its inner lip bearing against said piston skirt and its outer lip bearing against said inner cylndrical wall surface of the cylinder, and the other of said rings having its inner lip bearing against said outer cylindrical wall of the boss and its outer lip bearing against said piston skirt, each ring being oriented so that the groove therein defined by its cross section communicates with said pressure fluid chamber.

11. A hydraulic jack comprising, in combination, a cylinder and a piston disposed in axially nested, stacked relation and adapted to be axially extended in response to the introduction of pressure fluid therebetween, said cylinder comprising a body having axially spaced base and head ends, said head end being provided with an annular groove of a depth extending axially from the head end surface toward the base end surface, said piston having a head overlying the head end of the cylinder and provided with an axially extending skirt of a depth substantially equal to the depth of said annular groove and slidably received therein to define a closed pressure fluid chamber, means for introducing pressure fluid into said chamber for axially extending said piston as aforesaid, and sealing means acting between the inner and outer edges of said piston skirt and the contiguous wall surfaces of said groove in the cylinder to prevent leakage of pressure fluid from said chamber, the axial extent of said head and skirt of the piston being substantially less than the diameter of said skirt.

12. The jack according to claim 11 wherein said inner and outer edges of the piston skirt are undercut to define annular retaining grooves with said contiguous wall surfaces of the cylinder groove, said sealing means comprising a pair of rings received in said retaining grooves, said rings being of U-shaped cross section.

References Cited UNITED STATES PATENTS 2,686,402 3/1954 Samuel 92 240 5 2,763,464 9/1956 Leonhardt 254-29 3,096,075 7/1963 Brown 254 29 3,153,052 11/1964 Biach 254-29 3,176,961 4/1965 Glass 254-29 FOREIGN PATENTS 10 893,567 4/1962 Great Britain.

WILLIAM FELDMAN, Primary Examiner.

MILTON S. MEHR, Exwminer. 15 

1. AN HYDRAULIC JACK COMPRISING AN ANNULAR CYLINDER, A PRESSURE FLUID CHAMBER IN THE HEAD END PORTION OF SAID CYLINDER, A PRESSURE FLUID INLET TO SAID CHAMBER, AN ANNULAR PISTON SLIDABLY DISPOSED IN SAID CYLINDER, A PORTION OF SAID PISTON BEING ACCESSIBLE OUTSIDE SAID CYLINDER BY WHICH THE JACKING ACTION IS EFFECTED, INNER AND OUTER HOLLOW SEALING RINGS OF U-SHAPED CROSS-SECTION BETWEEN SAID PORTION AND THE HEAD OF SAID PISTON RESPECTIVELY SURROUNDING THE INNER AND OUTER WALLS OF SAID PISTON AND IN SLIDING ENGAGEMENT WITH THE INNER AND OUTER WALLS OF SAID ANNULAR CYLINDER INTERIOR, AND A CIRCULAR COVER PLATE FIXED TO THE 